From calculating my Jetbook which had a 3.6V battery, 800mAh,which ran 24hours, it uses 0.12W on average.

Most of the time the system is in standby while the display uses some power.

I think the 5W is well overestimated! A 1W will be more than enough. The problem is you need more when the device tries to charge at the same time.
But when you are not using the reader, it might charge from the 1W solar panel, while when you are using it the 1W is not enough to charge, but it is enough to provide the device with energy.
That way, unless you're reading/writing a lot of data to the card, while flipping pages like crazy, you'll never run out of battery.

That'd be what I'd think... The device itself uses very little battery. If you have a lite version where you can remove the batteries, or where you can direct the device to use USB power but not charge, you could perhaps run the device without batteries.
I say USB because I had hoped the solar panel could plug straight into the USB port!